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d1462d8558
Implement RFC 2580: Pointer metadata & VTable RFC: https://github.com/rust-lang/rfcs/pull/2580 ~~Before merging this PR:~~ * [x] Wait for the end of the RFC’s [FCP to merge](https://github.com/rust-lang/rfcs/pull/2580#issuecomment-759145278). * [x] Open a tracking issue: https://github.com/rust-lang/rust/issues/81513 * [x] Update `#[unstable]` attributes in the PR with the tracking issue number ---- This PR extends the language with a new lang item for the `Pointee` trait which is special-cased in trait resolution to implement it for all types. Even in generic contexts, parameters can be assumed to implement it without a corresponding bound. For this I mostly imitated what the compiler was already doing for the `DiscriminantKind` trait. I’m very unfamiliar with compiler internals, so careful review is appreciated. This PR also extends the standard library with new unstable APIs in `core::ptr` and `std::ptr`: ```rust pub trait Pointee { /// One of `()`, `usize`, or `DynMetadata<dyn SomeTrait>` type Metadata: Copy + Send + Sync + Ord + Hash + Unpin; } pub trait Thin = Pointee<Metadata = ()>; pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {} pub const fn from_raw_parts<T: ?Sized>(*const (), <T as Pointee>::Metadata) -> *const T {} pub const fn from_raw_parts_mut<T: ?Sized>(*mut (),<T as Pointee>::Metadata) -> *mut T {} impl<T: ?Sized> NonNull<T> { pub const fn from_raw_parts(NonNull<()>, <T as Pointee>::Metadata) -> NonNull<T> {} /// Convenience for `(ptr.cast(), metadata(ptr))` pub const fn to_raw_parts(self) -> (NonNull<()>, <T as Pointee>::Metadata) {} } impl<T: ?Sized> *const T { pub const fn to_raw_parts(self) -> (*const (), <T as Pointee>::Metadata) {} } impl<T: ?Sized> *mut T { pub const fn to_raw_parts(self) -> (*mut (), <T as Pointee>::Metadata) {} } /// `<dyn SomeTrait as Pointee>::Metadata == DynMetadata<dyn SomeTrait>` pub struct DynMetadata<Dyn: ?Sized> { // Private pointer to vtable } impl<Dyn: ?Sized> DynMetadata<Dyn> { pub fn size_of(self) -> usize {} pub fn align_of(self) -> usize {} pub fn layout(self) -> crate::alloc::Layout {} } unsafe impl<Dyn: ?Sized> Send for DynMetadata<Dyn> {} unsafe impl<Dyn: ?Sized> Sync for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Debug for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Unpin for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Copy for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Clone for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Eq for DynMetadata<Dyn> {} impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Ord for DynMetadata<Dyn> {} impl<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn> {} impl<Dyn: ?Sized> Hash for DynMetadata<Dyn> {} ``` API differences from the RFC, in areas noted as unresolved questions in the RFC: * Module-level functions instead of associated `from_raw_parts` functions on `*const T` and `*mut T`, following the precedent of `null`, `slice_from_raw_parts`, etc. * Added `to_raw_parts` |
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